Biomass · Combined heat and power
Every system in this hub throws away far more energy as heat than it delivers as power. A thermoelectric module rejects around ninety-five percent of what crosses it. A Stirling engine makes several units of heat for every unit of electricity. An engine on wood gas or biogas heats its own jacket and its own exhaust.
Whether that heat lands somewhere useful is the difference between a system worth building and an expensive way to burn wood.
Reading the efficiency claim
Electrical efficiency is the share of fuel energy that leaves as electricity. For the systems on this hub it runs from a few percent to something in the low twenties at best. Total system efficiency counts recovered heat as useful output too, and it can reach far higher figures because almost all fuel energy ends up as either power or heat.
Both are true. The second is quoted constantly and the first is what determines whether you get to run the lights, so the two get conflated in almost everything written about this subject.
The honest test is not which number is larger. It is whether the heat counted in the larger number is heat you actually wanted.
A worked distinction
Two identical machines burn the same wood. One sits in a heated workshop in February, and its reject heat displaces fuel that would otherwise have been burned for exactly that purpose. Its heat is worth what the displaced fuel cost.
The other sits outdoors in July, venting the same heat to the sky. Its total system efficiency on paper is unchanged. Its actual efficiency is the electrical figure alone, because nothing received the heat.
The number that governs a real installation is not the specification. It is the fraction of annual reject heat that a load was waiting for at the moment it arrived.
What each system rejects
Grade matters as much as quantity. Heat arriving at a high temperature can do anything cooler heat can do and more. Low-grade heat is plentiful and awkward, and most of what these systems reject is low grade.
Roughly ninety-five percent of everything crossing the module arrives at the cold plate, which must be kept cold for generation to continue at all. The heat is abundant and at a low temperature by design, which suits preheating and suits little else.
Several units of heat per unit of electricity, delivered through a cooling jacket designed from the start to feed a water loop. Of everything here this is the most straightforward heat to capture, because the machines were built as heating appliances.
Two separate streams. The cooling jacket gives moderate-temperature heat that is easy to plumb. The exhaust is much hotter and more useful, and also more difficult, because exhaust heat exchangers foul and corrode in ways jacket loops do not.
A digester is a heat consumer as well as a gas producer, holding its contents near body temperature so the bacteria keep working. Engine reject heat going back into the vessel is the reason the whole arrangement can work at all in a cold climate.
Sizing
This is the one design rule on the page, and it inverts what most people assume. Environmental Protection Agency guidance is that combined heat and power systems are typically designed and sized to meet a site's year-round baseload thermal demand, and that the economic value of the system is maximised when all of both the electrical and thermal output can actually be used[1].
Baseload is the operative word, meaning the demand that is always there rather than the peak. Year-round is the other one, and on most properties it is the harder test of the two.
Size against your electrical load instead and you get a machine producing more heat than the property can absorb for most of its running hours. The surplus is dumped, and a system whose heat is dumped is simply a generator with poor efficiency and extra plumbing.
The machine runs when heat is wanted, and whatever electricity comes out is taken as it comes. Fuel is used well, because everything produced is used.
The consequence is that electrical supply is not under your control. It arrives on the heating system's schedule, which is why batteries belong in the design rather than as an afterthought.
The machine runs when power is wanted, and the heat is used if something needs it and dumped if not. Electrical supply becomes predictable.
The consequence is that the efficiency case largely disappears during any period when the heat has nowhere to go, and with it most of the reason to have chosen this over a generator.
Work the thermal side the same way this domain works everything else, from measured demand rather than assumption, and set the electrical side against your own load list afterwards rather than first.
The seasonal problem
A household's heat demand collapses in summer. Its electricity demand does not, and in a hot climate it rises. A system sized against January has nowhere to put its heat for months at a time, and running it regardless means dumping most of the fuel energy to the air.
This is the central difficulty of combined heat and power on a property rather than at a factory, and it is why the year-round part of the sizing rule does so much work. Two consequences follow, and both are worth designing around from the start.
Domestic hot water carries more weight than space heating
Space heating is large and seasonal. Hot water is smaller and constant, and a household needs it in August exactly as in February. In an annual accounting it is often the only thermal load that keeps a summer running hour honest.
It is also the load that introduces the water hygiene requirements in the next section, so it is not a free win. It is a real one that comes with obligations.
A buffer tank lets a machine run at a sensible output and release the heat over the following hours, which smooths a day. No practical domestic tank stores February heat until July, and any plan that assumes otherwise has an arithmetic problem rather than an engineering one.
Somewhere for unwanted heat to go is not an optimisation. A system with no path for surplus heat can overheat its own loop, and the arrangement for shedding it should be part of the design rather than something added after the first hot week.
Safety
Recovered heat is frequently low grade, and the obvious use for low-grade heat is preheating domestic hot water. That is a sound idea with a specific hazard attached, and it is almost never mentioned in writing about small combined heat and power.
A preheat tank warmed by reject heat can sit for long periods at exactly the temperatures that suit the organism you least want in a water system.
The Centers for Disease Control identifies 77 to 113 degrees Fahrenheit as the range most favourable to Legionella growth, and notes it can grow at temperatures as low as 68. Its control guidance is to store hot water above 140 degrees Fahrenheit and to keep circulating hot water above 120[2].
A tank fed only by low-grade reject heat may never reach any of those figures. It needs a means of being brought to full storage temperature by the primary heater, not left wherever the recovered heat happened to leave it.
Storing at that temperature then creates a scalding risk at the taps, which is why CDC guidance pairs it with thermostatic mixing at or near the point of use, and notes that state anti-scald rules sometimes cap temperatures below what is needed to limit growth. That tension is real and is resolved with mixing valves, not by lowering the tank.
Engine jacket water, digester heating and anything carrying glycol should never share water with what comes out of a tap. A heat exchanger between them is standard practice and is a requirement rather than a refinement.
A vented low-temperature hydronic loop is ordinary plumbing. A sealed vessel making steam is a different regulated object with its own inspection regime, and crossing that line by accident is how heat recovery becomes a pressure vessel problem.
Plumbing that touches potable water is governed by your local plumbing code and inspected by your authority having jurisdiction. Backflow prevention and cross-connection control are the parts most often got wrong by people who plumb their own heating, and they are the parts that affect everyone drinking the water.
Common mistakes
The most consequential error, and it produces a machine that dumps heat for most of its life. Thermal baseload sets the size, and the electrical output is whatever that size gives you.
Annual efficiency depends on coincidence, not on capacity. Heat delivered to a house that is already warm has the same value as heat vented outdoors, and both should be counted the same way.
A buffer tank moves heat across hours. It does not move heat across months, and a plan that quietly relies on it doing so has not been worked through.
Low-grade recovery lands water squarely in the growth range. A preheat tank needs a path to full storage temperature and mixing at the fixtures, and neither is optional because the heat was free.
Every system needs somewhere for heat to go when the load disappears, including in the middle of a failure. Designing it in is straightforward, and retrofitting it after an overheat is not.
On a property already burning wood, the fuel was being spent regardless. The honest comparison is against simply heating with that wood and buying the electricity, which is often cheaper than it looks.
Next
Everything covered so far can be understood and acted on directly. The next three guides cover wood gasification, biogas digesters and small-scale steam, and those carry risk that sits in the design rather than in carelessness.
They are covered here in full, and they stop short of build instructions and operating procedures. The hub explains that standard and why it applies to those three and not to these four.
Sources
Efficiency figures on this page are ranges for understanding the trade rather than design values. Any work touching potable water is governed by your local plumbing code and inspected by your authority having jurisdiction, and backflow prevention and cross-connection control are not areas to work out by yourself. No figure here is taken from a company selling the equipment it describes.
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